Biaxially oriented polyester films are used for packaging, industrial, electronic, decorative, and label applications and often perform multiple functions. In particular, biaxially oriented PET films and laminations are popular, high performing, and cost-effective flexible substrates for a variety of snack food packaging applications. Such packaging films must sometimes perform in a lamination to provide printability, transparent or matte appearance, or slip properties; they sometimes must provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties; they sometimes must provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
In recent years, interest in “green” packaging has been strongly developing. Packaging materials based on biologically derived polymers are increasing due to concerns with non-renewable resources, raw materials, and greenhouse gas generation. Bio-based polymers are derived from renewable or sustainable sources such as plants. These polymers are believed—once fully scaled-up—to reduce reliance on petroleum, reduce production of greenhouse gases. Bio-based polymer such as polylactic acid (PLA)—which is currently derived from corn starch—is one of the more popular and commercially available materials that can be used for packaging film applications.
For such a bio-based polymer to be fit-for-use for many snack food packaging applications, it is desirable that the bio-based polymer films match as many of the attributes possible of conventional packaging materials, for example, conventional multi-layer biaxially-oriented PET. Desirable attributes of conventional PET that are preferably met include thermal and dimensional stability, heat sealability, printability, controlled coefficient of friction (COF), metallizability, gas transmission barrier, etc.
In particular, for high barrier packaging, metallized bio-based oriented films should demonstrate good oxygen and moisture barrier properties. For example, in the case of a metallized oriented PLA, good oxygen barrier properties are generally easily achieved due to the polar nature of PLA, which provides good hydrogen-bonding of the polymer molecules. However, this polar nature tends to be detrimental for achieving high moisture barrier. Without being bound by any theory, the thought is that water molecules—being polar themselves—may more easily migrate through a polar polymer film than a non-polar polymer film. In addition, it is possible that the PLA substrate can absorb moisture and swell, thus changing the physical and dimensional properties of the PLA substrate. In particular, such swelling—especially at the interface between the vapor-deposited metal in a metallized PLA film and the adjacent PLA substrate surface—can cause morphological and dimensional changes in which the relatively inflexible inorganic metal layer can not conform to. This can then result in cracking of the metal layer and attendant loss of gas barrier properties. Biaxially oriented PET (BOPET) films, however, exhibit excellent oxygen gas barrier properties and reasonably good moisture barrier properties due to its high Tg, crystallinity, and aromatic structure. BOPET is less subject to swelling and dimensional changes due to moisture, although it does share some of the sensitivity to moisture that biaxially oriented PLA (BOPLA) does due to its polar nature.
There are other issues inherent with bio-polymers such as PLA used in flexible packaging applications. BOPLA typically has lower thermal resistance and higher heat shrinkage than BOPET which can be a problem in downstream processes seen in converting such as drying temperatures after printing or coating, extrusion lamination, and metallizing. In fact, the high thermal resistance and dimensional stability of BOPET films is what make them useful and attractive in many applications. In addition, from an end-user stand-point, the high stiffness of BOPLA packaging can make it prone to dead-fold issues whereby the package can be easily creased, causing a shelf-worn appearance; and furthermore, the BOPLA package can be much noisier than a BOPET package which can be a complaint from the consumer.
However, if BOPET packaging can be made from bio-based sources instead of petroleum sources, this would solve the converting, end-user, and consumer concerns that BOPLA packaging entails, while reducing reliance on petroleum, reducing overall potential carbon footprint, and all the packaging to be produced from a sustainable resource (plants). The only drawback would be that unlike BOPLA or some other bio-polymers, BOPET would not be compostable or degradable without modifying additives. However, without being bound by any theory, a bio-based/sourced BOPET could be a way to sequester carbon dioxide from the atmosphere as the source plant material could take in CO2 from the atmosphere which is then converted to polyesters such as polyethylene terephthalate and then converted to polyester packaging which does not degrade and return CO2 to the atmosphere.
Coca-Cola Company's US Publication No. 20090246430A1 states that “It is known in the art that carbon-14 (C-14), which has a half life of about 5,700 years, is found in bio-based materials but not in fossil fuels. Thus, ‘bio-based materials’ refer to organic materials in which the carbon comes from non-fossil biological sources. Examples of bio-based materials include, but are not limited to, sugars, starches, corns, natural fibers, sugarcanes, beets, citrus fruits, woody plants, cellulosics, lignocellulosics, hemicelluloses, potatoes, plant oils, other polysaccharides such as pectin, chitin, levan, and pullulan, and a combination thereof.” The detection of C-14 is indicative of a bio-based material. C-14 levels can be determined by measuring its decay process (disintegrations per minute per gram carbon or dpm/gC) through liquid scintillation counting. This reference teaches the use of bio-based ethylene glycols and terephthalic acids to form a bio-based polyethylene terephthalate resin useful for beverage bottles.
US Publication No. 20100028512A1 describes a method of producing bio-based polyester terephthalate (PET) resin which may then be used to make articles, containers, or packaging for food and beverage products. The application also discloses the use of bio-based polyethylene to produce closures, caps, or lids for bio-based PET containers as well as the use of bio-based polyethylene labels via film extrusion for the containers. However, there is no contemplation of producing bio-based polyethylene terephthalate films for packaging applications.
It is the objective of this invention to provide a method for producing useful films and laminations using bio-based polyethylene terephthalate homopolymers and copolymers for various packaging applications. Such bio-based polyester articles will contain a certain amount of 14C-isotope, a quantity that is thus distinguishable from petroleum-based polyesters. These bio-based polyesters are made from, in turn, bio-based monomers, which are derived from plant-based intermediates such as alcohols and sugars.